Pressure Cap Description

Caution: Refer to Pressurized Cooling System Caution in Service Precautions.

The surge tank uses a plastic pressure-vent cap to allow a build up of pressure in the cooling system. The cap allows the cooling system to operate at a higher than atmospheric pressure. This pressure raises the boiling point of the engine coolant.
The pressure-type cap contains a blow off (pressure) valve and a vacuum (atmospheric) valve. The cap protects the radiator by relieving any pressure that exceeds design limits.
In order to prevent radiator collapse, caused by vacuum as the engine cools, the vacuum valve prevents excessive pressure differences from occurring between the cooling system and the atmosphere.
The surge cap has a threaded design to allow for easy and safe removal. Do the following in order to safely remove the cap from a cool system:

Caution: Refer to if there is evidence of steam, do not open in Service Precautions.

1. Slowly rotate the cap counterclockwise 1/4 turn and then stop, do not press down.
2. Allow any residual pressure, indicated by a hissing sound, to be relieved.
3. After the hissing stops, continue to rotate the cap counterclockwise until the cap is removed.
4. Retighten the cap using hand tight pressure.